Butadiene is an important starting material for the production of high molecular weight polymers and is used extensively to form synthetic rubber including styrenebutadiene rubber, nitrile-butadiene rubber, buna-S rubber, and trans-polybutadiene rubber, and adiponitrile and styrene butadiene latex in paints. Butadiene is usually a by-product from steam cracking naphtha and is contained in a crude C.sub.4 stream. However, the crude C.sub.4 stream regularly contains other hydrocarbons that must be removed before the butadiene may be used as a starting material. The principal components of the crude C.sub.4 stream are butanes, butenes, butadienes, and acetylenes including ethylacetylene, methylacetylene and vinylacetylene. Historically, the hydrocarbons are removed from the butadiene in two steps. First the butanes and butenes are removed by extractive distillation using a polar solvent that has greater affinity for unsaturated hydrocarbon compounds than for saturated compounds. Then two different approaches have been used for the removal of acetylenes: extractive distillation using a solvent to selectively absorb the acetylenes or selective hydrogenation of the acetylenes; see Abrevaya, H; Vora, B. V; Lentz, R. A., "Improved Butadiene Technology for Naphtha Cracking," Presented at the Fifth World Congress of Chemical Engineering, San Diego, July 1996. Examples of selective hydrogenation include U.S. Pat. No. 3,342,891, U.S. Pat. No. 3,541,178, U.S. Pat. No. 3,842,137, U.S. Pat. No. 3,898,298, U.S. Pat. No. 4,277,313, U.S. Pat. No. 4,469,907, U.S. Pat. No. 4,704,492, U.S. Pat. No. 5,414,170, and GB 2 040 995.
The patents U.S. Pat. No. 3,541,178, U.S. Pat. No. 3,842,137, U.S. Pat. No. 4,469,907, and U.S. Pat. No. 4,704,492 provide alternate methods of introducing hydrogen to the selective hydrogenation reactor. U.S. Pat. No. 3,342,891 teaches a process of fractionating the butadiene-containing stream into two portions with one portion enriched in acetylenes. Only that portion enriched in acetylenes is subjected to selective hydrogenation. After selective hydrogenation the two portions are recombined. U.S. Pat. No. 5,414,170 discloses a process for selectively hydrogenating the acetylenes in an olefin plant process stream downstream of a front end depropanizer and upstream of further separation zones. U.S. Pat. No. 3,898,298 discloses selective hydrogenation of vinylacetylene using palladium on alumina catalysts at 35.degree. C. and 7 atmospheres to achieve mixed phase operation. GB 2 040 995 discloses admixing a recycle stream and a fresh C.sub.4 stream and hydrogenating the mixture, fractionating the product, feeding back an acetylenic stream and recovering butadiene. U.S. Pat. No. 4,277,313 discloses first selectively hydrogenating C.sub.4 -alkyne components and then using extractive distillation to separate a 1,3-butadiene-rich selective solvent extract phase. Others have used catalytic distillation to concurrently hydrogenate both the dienes and acetylenes; see DE 19520389, EP 733,030, and WO 9404477.
The present invention integrates the two steps of separating the butanes and butenes by extractive distillation and hydrogenating the acetylenes into a single unit by using catalytic extractive distillation. The separation of butanes and butenes from the butadienes is accomplished by extractive distillation, but in the present invention the extractive distillation unit has a reaction zone that contains a catalyst effective for the hydrogenation of acetylenes, hence the term catalytic extractive distillation. The catalyst in the reaction zone is specifically chosen to minimize the hydrogenation of butadienes. As is the case with the butanes and butenes, acetylene hydrogenation products other than butadiene are separated from the butadienes by extractive distillation. Integrating the two steps of the process into a single unit provides a marked reduction in both capital costs and operation costs.